Ultrasonic tracking of ultrasound transducer(s) aboard an interventional tool

ABSTRACT

In one aspect, an ultrasound receive beamformer is configured for one-way only beamforming of transmissive ultrasound using one-way delays. The receive beamforming in some embodiments is used to track, in real time, a catheter, needle or other surgical tool within an image of a region of interest. The tool can have embedded at its tip a small ultrasound transmitter or receiver for transmitting or receiving the transmissive ultrasound. Optionally, additional transducers are fixed along the tool to provide the orientation of the tool.

CROSS-REFERENCE TO PRIOR APPLICATIONS

This application is a Continuation application of U.S. Ser. No.14/958,355, filed on Dec. 3, 2015, now U.S. Pat. No. 9,585,628, whichwill issue on Mar. 7, 2017, which is a Continuation application of U.S.Ser. No. 13/643,374, filed on Oct. 25, 2012, now U.S. Pat. No.9,282,946, issued on Mar. 15, 2016, which is the U.S. National Stageapplication under 35 U.S.C. § 371 of International Application No.PCT/IB2011/051729, filed on Apr. 20, 2011, which claims the benefit ofU.S. Provisional Patent Application No. 61/330,641, filed on May 3,2010. These applications are hereby incorporated by reference herein.

FIELD OF THE INVENTION

The present invention is directed to one-way only beamforming oftransmissive ultrasound and, more particularly, to using suchbeamforming for tracking an interventional tool during real-timeimaging.

BACKGROUND OF THE INVENTION

Precise visualization of catheters and real-time knowledge of theirlocalization with respect to the anatomy are needed for minimallyinvasive interventions. Intra-operative ultrasound is often used forthese purposes. However, many surgical tools are difficult to image withconventional pulse-echo ultrasound. For instance, the usability of 3DTransoesophagial Echocardiography (3D-TEE) for guidance of cathetercardiac interventions is still limited because it is challenging toimage catheters reliably with ultrasound. Catheters are specularreflectors that reflect the sound away from the imaging probe if theinsonifying angles are not favorable. As a consequence, a catheterappears on and off on 3D-TEE images during its progression through thecardiac chambers. It also frequently happens that some parts of thecatheter are visible and others not depending on the local angle betweenthe catheter and the imaging beams, for instance the distal end of thecatheter may be invisible and some point along its shaft may be mistakenas its tip. Also, due to weak reflection, signal from the catheter maybe drowned in signal from the surrounding anatomy.

Electromagnetic (EM) tracking sensors have been mounted aboard cathetersfor tracking their tip and other selected locations along their shaft.However, the positioning accuracy of such sensors can get really poor(of the order of 10 mm) in EM distorted operating environment.Additionally, an independent EM tracking system adds to the equipmentcost and clutter in the catheter laboratory.

U.S. Pat. No. 4,249,539, entitled “Ultrasound needle tip localizationsystem,” to D. H. R. Vilkomerson et al. (hereinafter “Vilkomerson”)discloses an active ultrasound transducer that is attached at the tip ofa biopsy needle to be imaged by a B-mode ultrasound scanner. Thetransducer at the tip of the needle, upon sensing signals from theimaging probe, immediately transmits back a short pulse. The ultrasoundtransducer on the needle thus merely acts as a “super-reflector” thatre-radiates a strong ultrasonic signal upon insonification. The imagingparadigm is not modified and the “super-reflector” is simply seen as avery bright point in the ultrasound image. Furthermore, all embodimentsdescribed in that patent results in a very bad lateral resolution of theneedle tip because there is no proper transmit focusing.

SUMMARY OF THE INVENTION

In one aspect, the present invention is directed to an ultrasonictracking device and method for reliably determining both the position(particularly, of the tip) and orientation of a catheter or othersurgical tool (e.g., a biopsy needle) relatively to the surroundinganatomy, in three dimensions and in real time. One or more smallultrasound transducers, serving as sources or receivers, are placed(embedded) at known locations on the catheter or surgical tool.

In the case of the ultrasound receiver, its 3D position can be obtainedby beamforming the signals received by it as the ultrasound beams sweepthe field of view during pulse-echo acquisition. In accordance with thistechnique, the ultrasound scanner scans using synthetic aperture withvirtual transducers. The effect is to allow sensitive and specificcatheter imaging, with perfect accuracy and the same resolution as thatof the ultrasound imaging scanner at the same depth.

In other embodiments, in which to ultrasound transducer(s) can be eithertransmitters or receivers, tracking and pulse-echo image acquisition areseparated by means of frequency or by means of timing (e.g.,alternating, or otherwise interspersing, imaging frame with trackingframes). For example, such a transmitter, in the case of timingseparation, may be triggered active in transmit by the line or frametrigger of an ultrasound scanner that temporarily interrupts itspulse-echo imaging of the medium of interest. Propagation of sound thenoccurs from the transmitter to the individual elements of the imagingarray. The transmitter can be precisely imaged by adjusting theultrasound scanner's beamforming delays to account for the one-way onlypropagation of transmissive ultrasound between the tracked ultrasoundtransmitter and the imaging array.

“Transmissive ultrasound,” as this term is used herein, refers toultrasound that has not reflected back for processing, in contrast tothe ultrasound echoes processed in pulse-echo imaging. With respect toaspects of what is proposed herein, transmissive ultrasound is emittedfrom the transmitter for reception by an ultrasound scanner for thoseembodiments having a transmitter, and is emitted by the scanner forreception by the receiver for those embodiments having a receiver.

“One-way only” receive beamforming or “one-way” receive beamforming is,as the term is used herein, receive beamforming that uses “one-way”beamforming delays. “One-way” beamforming delays are, as the term isused herein, beamforming delays that account for the duration ofultrasound propagation toward the transducer based upon whose output thebeamforming is performed. This is in contrast to “two-way” or“pulse-echo” beamforming which uses two-way delays, i.e., a delay forthe pulse and a delay for the echo. With respect to aspects of what isproposed herein, the transducer, based upon whose output the beamformingis performed, is the ultrasound scanner for those embodiments having atransmitter, and is the receiver for those embodiments having areceiver.

In accordance with an aspect of the present invention, an ultrasoundreceive beamformer is configured for one-way only beamforming oftransmissive ultrasound using one-way delays.

In accordance with a related aspect, the ultrasound receive beamformeris configured for three-dimensional imaging.

In a further aspect, an ultrasound device comprising the beamformer isconfigured for interspersing acquisition of imaging frames, upon whichtwo-way beamforming is performed, with acquisition of tracking frames,upon which the one-way only beamforming is performed.

In accordance with another related aspect, the ultrasound receivebeamformer is configured to, by the beamforming, localize an objectwithin a region of interest.

In yet another aspect, for an ultrasound device comprising thebeamformer, the object is an ultrasound transducer serving as a sourceof the transmissive ultrasound.

In a yet further aspect, the ultrasound device is configured for anultrasound scanner triggering, by a line trigger or by a frame trigger,emission of sound from the source and/or for the source triggering thescanner active for image acquisition.

As another, related version, for an ultrasound device comprising theultrasound receive beamformer, hardware that senses ultrasound forperforming the one-way only beamforming is that which senses ultrasoundused in pulse-echo imaging of the region of interest.

In one further aspect, an ultrasound device includes an ultrasoundreceive beamformer, hardware that senses ultrasound for performing theone-way only beamforming being separate, and physically apart from, thatwhich senses ultrasound for performing receive beamforming used inpulse-echo imaging of the region of interest.

As a yet further aspect, an ultrasound device comprises the ultrasoundreceive beamformer, and a plurality of tracking transducers thatincludes the object within the region of interest, each of the pluralityserving as a source, or each serving as a receiver, of the transmissiveultrasound, each being attached to an interventional tool. The pluraltracking transducers are located mutually apart for real-timedetermination, by the device, of an orientation of the tool. Ones, ifany, of the plural tracking transducers that serve as a source areconfigured for emitting signals that allow them to be distinguishablefrom each other.

In an alternative aspect, an ultrasound device comprises the beamformerconfigured for real-time tracking.

In another aspect, the ultrasound device is configured forsuperimposing, in real time, tracking frames on imaging frames.

In one other aspect, ultrasound for creating said tracking frames isissued by a synthetic aperture technique.

In a yet further aspect, the ultrasound device is configured forsuperimposing, on an imaging frame, a tracking frame having a differentcolor map.

In some versions, the ultrasound device further comprises an ultrasoundreceiver subject to the real-time tracking.

In particular versions, the ultrasound receive beamformer is configuredas a retrospective dynamic transmit (RDT) receive beamformer for theone-way only beamforming, the transmissive ultrasound being issued bysynthetic aperture with virtual transducers.

In a related aspect, an ultrasound device comprises the RDT receivebeamformer, an ultrasound transducer and an ultrasound scanner, thetransducer serving as a receiver of the transmissive ultrasound andbeing disposed within a region of interest subject to imaging by thescanner.

As one other aspect, a method for receive beamforming of transmissiveultrasound includes configuring a receive beamformer for one-way onlybeamforming of the transmissive ultrasound using one-way delays.

In an additional aspect, the method further comprises one or both of:

-   -   configuring into different frequencies the transmissive        ultrasound and pulse-echo ultrasound; and    -   alternating acquisition of imaging frames with acquisition of        tracking frames.

In a different, but related, aspect, a device configured for localizingat least one of an ultrasound transmitter and an ultrasound receiver,disposed within a region of interest, is configured for:

-   -   issuing transmissive ultrasound from at least one of:        -   the transmitter to an ultrasound scanner for imaging the            region of interest; and        -   the scanner to the receiver; and    -   one-way only beamforming the received transmissive ultrasound        which, from pulse-echo imaging ultrasound, is separate by        frequency or by timing, or which is issued by synthetic aperture        with virtual transducers.

In a further, related version, an ultrasound device is configured for,automatically and without user intervention, calculating, with respectto an ultrasound scanner, an imaging depth of an ultrasound receiver,for switching transmit focal depth to the imaging depth, and for issuingimaging beams with that imaging depth as their focal depth. The deviceis further configured for using output of the receiver to localize thereceiver.

Details of the novel ultrasonic tracking device and method are set forthfurther below, with the aid of the following drawings, which are notdrawn to scale.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual diagram offering a comparison between two-waybeamforming and one-way only beamforming;

FIG. 2 is a schematic diagram depicting a configuration, which usessynthetic aperture with virtual transducers, in which a receivingultrasound transducer fixed to a catheter is disposed within a region ofinterest

FIG. 3 is a conceptual diagram that portrays a synthetic apertureacquisition scheme in the top drawing, and the same scheme using virtualtransducers in the bottom drawing;

FIG. 4 is a schematic diagram showing the ultrasound transducer as atransmitter;

FIG. 5 is a schematic diagram showing the received signals at thetracked receiver are fed back to the ultrasound scanner's beamformingmodule and one-way beamforming is performed;

FIG. 6 is a flow chart showing an embodiment in which transmit focaldepth is switched to the measured image depth of the receiver; and

FIG. 7 shows an interventional tool with two embedded transmitters.

DETAILED DESCRIPTION OF EMBODIMENTS

FIG. 1 offers, by way of illustrative and non-limitative example, acomparison between two-way beamforming and one-way only beamforming. Thetop figure, representative of two-way beamforming shows an imaging array102 of N elements 104 issuing ultrasound that impinges on a reflector106. Since the ultrasound waves go out and back (from the imaging arrayto the reflectors and back to the imaging array), we talk of “two-way”or “round-trip” beamforming. On receive (of the ultrasound that hasreflected back), beamforming determines the reflectivity of thereflector 106 and the position of the reflector relative to the array102. The array 102 send out a beam 108 that reflects off reflector 106and returns to all elements 104 of the array 102. The flight of thepulse is over a distance r(P)+d(i,P) for element i. Each element 104measures continually the amplitude of the return ultrasound. For eachelement 104, the time until a maximum of that measurement, i.e., the“round-trip time of flight,” is indicative of the total flight distance.Since the r(P) leg of the flight is constant, the return flight distanced(i,P) is determined. From these measurements, the relative position ofthe reflector 106 is computed geometrically. As to the reflectivity ofthe reflector 106, it can be indicated by summing the maxima over all i(i.e., over all elements 104).

As seen from the bottom figure, one-way only (receive) beamforming,there is no echo. Instead, as in the case of transmitter 110, it emits apulse 112 which is incident on each element 104 of the array 102. Theflight here, in contrast to the two-way beamforming case, is over thedistance d(i,P). The time from emission of the pulse 112 until themaximum amplitude reading at an element 104 determines the value d(i,P)for that element i. Thus, the position of the transmitter 110 can bederived geometrically, and the reflectivity calculated by summing themaximum amplitude readings.

Although one-way beamforming is implementable in the time domain viadelay logic, as discussed hereinabove, it can also be implemented in thefrequency domain by well-known Fourier beamforming algorithms.

FIG. 2 depicts a configuration, which uses synthetic aperture withvirtual transducers, in which a receiving ultrasound transducer 202,fixed to a catheter 204 (or other interventional tool or instrument,flexible or rigid), is disposed within a region of interest 206 (such aspart of the heart of a patient or animal subject) which is subject toimaging by the ultrasound scanner. While the region of interest 206 isbeing imaged by the scanner, whose imaging probe 208 is shown in thefigure, output 210 of the receiver 202 of the transmissive ultrasound isone-way only beamformed by a beam-space beamformer 212 which imagesusing synthetic aperture with virtual transducers, i.e., a techniqueused in retrospective dynamic transmit focusing (RDT).

The front end of the scanner guides transmit and receive beamformingfrom the imaging probe 208 used, for example, in TEE (TransesophagealEchocardiography). For example, the receiver(s) 202 aboard the catheterare triggered active in receive (to, in other words, start the clock attime zero in measuring the one-way delay) by the scanner's line trigger209 (a trigger signal is emitted each time the TEE probe emits adifferent transmit beam). For this purpose, an electrical connection ismade to the receiver 202, such as a wired cable extending from thescanner up through the catheter 204 and to the receiver.

In practice, what is proposed herein is of particular value inthree-dimensional imaging, for this and all embodiments described below.

The scanner's beamformer 214 processes the beamformed signal or displayas a tissue image 216.

The signals received by the receiver(s) 202 aboard the catheter 204 aresent to the RDT beamformer 212. Moreover, hardware 218, i.e.,(transducer hardware) that senses ultrasound for performing the one-wayonly beamforming is separate, and physically apart from, hardware 220,including, for example, transducer elements of an imaging array, whichsenses ultrasound for performing receive beamforming used in pulse-echoimaging of the region of interest 206. As mentioned above, the receivebeamforming is one-way only beamforming of the transmissive ultrasoundusing one-way delays.

Output of the RDT beamformer 212 is fed back to the back end of theultrasound scanner for processing and displaying the resulting image222.

Advantageously, beamforming localizes the ultrasound receiver in theregion of interest 206 and yields an image of the receiver in the samecoordinate system as the regular pulse-echo image of the region ofinterest, i.e., anatomy.

The receiver image 222 can be conveniently superimposed onto the image216 of the anatomy, as seen in the figure from the overlay image 224,as, for example a grayscale image with full dynamic range, or,alternatively an icon representative of the image, which here is animage of the tip of the catheter 204, superimposed on the image 216 ofthe anatomy from the regular pulse-echo imaging sequence. With regard toan icon or predefine drawing (such as a cross or star), it can be placedat the location of maximum intensity in the tracking frame 222. If theicon is given, for example, a solid color, its color map is constant atthat color; whereas, the underlying image of anatomy has a differentcolor map corresponding, for example, to the grayscale used.

In particular, the dynamic range of the tracking frames 222 is ideallyhalf that of the imaging frames 216 to take into account one-waybeamforming only that induces sidelobes roughly twice as high asconventional two-way imaging. The tracking frames 222 are superimposedon the imaging frames 216 in real-time; they are ideally displayed with,as mentioned above, a different color. This allows unequivocalidentification of the tracked device 218, and avoids saturation of thebrightness image with potentially very strong signals received from theactive source (as compared to the weaker pulse-echo signalsbackscattered by the tissue). Optionally, the point with maximumbrightness in the tracking frames is simply isolated and taken as theneedle tip location. A schematic drawing of the needle tip at thecalculated location can then be superimposed on the image of theanatomy. The calculated position of the tracked receiver 218 can besuperimposed on the real-time intra-operative ultrasound imaging displayand/or pre-operative co-registered CT or MR images. Note that, if a 2Dultrasound probe is used allowing 3D tracking of the needle or catheter204, 3D one-way beamforming can be performed during the tracking frames222 even if the displayed pulse-echo imaging frames are 2D for easiervisualization. It allows seeing the interventional tool 204 even when itis out of the imaging plane, and knowing its position with respect tothe current imaging plane, enabling tool and imaging guidance.

Alternatively, although not shown in FIG. 2, the beam-space beamformer212 may be part of the scanner's beamformer 214, i.e., signals from thereceiver(s) 202 may be fed back to the scanner with the scanner'sbeamformer 214, in this case, having a separate one-way only beamformingfunction for the fed back signals.

FIG. 3 portrays a synthetic aperture acquisition scheme in the topdrawing, and the same scheme using virtual transducers in the bottomdrawing. Both schemes are utilized in aspects of the invention, althoughthe one with virtual transducers (the RDT embodiment) is the oneillustrated in FIG. 2.

Turning now to the top drawing in FIG. 3, the N elements of the imagingarray sequentially send out an impulse, i.e., pulse, into the medium.Let ri,P(t) be the temporal signal received by the receiver P in themedium when element i fires an impulse. (The origin of time is takeneach time an element is fired.) The travel time from i to P ist _(i,P) =d(i,P)/c  (equation 1)where d(i,P) is the distance between element i and receiver P, and c isthe medium's speed of sound. Thus r_(i,P)(t) has its maximum at t_(i,P).An image of the receiver in space is formed by, for each point Q insidethe field of view, taking the summation:s(Q)=Σr _(i,P)(t _(i,Q))  (equation 2)over i=1 to N. Apodization functions may optionally be used as isstandard practice in the art.

The quantity s(Q) will be maximized for Q=P; that is, at the location ofthe receiver.

Referring now to the bottom drawing of FIG. 3, the RDT with virtualtransducers scheme is similar to above-described synthetic aperturescheme—the imaging array is replaced by a “virtual array” made of“virtual elements.” Each virtual element is the focal location of onefocused beam emanating from the real (physical) imaging array. There areas many virtual elements as there are focused beams from the imagingarray. The imaging array sends out N beams into the medium, sweeping thefield of view. Let r_(i,P)(t) be the temporal signal received by thereceiver P in the medium when the beam number i is fired into the medium(i.e., the virtual element i emits an impulse). The origin in time isnow taken when the beam is emitted. The travel time from virtual elementi to P ist _(i,P) =d(i,P)/c  (equation 3)The time it takes for the transmitted beam to focus at the location ofthe virtual transducer ist _(i) =d(i)/c  (equation 3)where d(i) is the distance between the center of the imaging array'sactive aperture and the focal point of transmit beam i (i.e., thevirtual transducer i). In usual transmit schemes, all transmits arefocused at the same depth, so d(i) does not depend on i; let us call itd₁ andt ₁ =d ₁ /c  (equation 4)It thus takes the time t₁+t_(i,P) between the emission of beam i andreception of the corresponding impulse at point P. The quantityr_(i,P)(t) thus has its maximum at t₁+t_(i,P).

An image of the receiver in space is formed by, for each point Q insidethe field of view, doing the summation:s(Q)=Σr _(i,P)(t ₁ +t _(i,Q))  (equation 2)over i=1 to N.

The quantity s(Q) will be maximized for Q=P which is the location of thereceiver.

In reality, since the virtual transducers are not punctual and have acertain directivity that is governed by the shape of the actuallytransmitted imaging beams, it is necessary, as known in the art, toperform some transmit beam simulations to compute the exact theoreticalarrival times of each beam i at each point Q.

The RDT beamformer 212 beamforms the data received by the ultrasoundreceiver in beam-space (like RDT), thereby affording optimal(diffraction-limited) resolution of the tracked object at all depths.

FIG. 4 shows the ultrasound transducer as a transmitter 402; however,the embodiments of the invention that are realizable with thetransmitter are alternatively implementable with a receiver instead,such as the receiver 202 discussed above.

In order to simplify the description, it is first assume that an activesource, i.e., the transmitter 402, is placed on the tracked surgicaltool 404. Because of reciprocity, the active source that sends signalstoward the ultrasound scanner can be replaced by an ultrasound receiverthat receives signals from the ultrasound scanner, without changing thesignal processing for its localization.

A small ultrasound “tracked” source, i.e., the transmitter 402, isplaced at the tip of the catheter, needle or other interventional tool404. Ideally, the tracked source 402 is as omnidirectional (monopolarradiation pattern) as possible in order to be able to sense signals fromit from any direction of space. The tracked source is able to emit shortpulses (optionally, more complicated waveforms with transmit codes)which ideally (but not necessarily) have a frequency band different fromthat of the imaging pulses of the intra-operative imaging ultrasound inorder to avoid interference between the tracking and imaging pulses.Reception of tracking and imaging pulses may be differentiated eithersimply with receive filters or more sophisticated pulse signatureidentification algorithms.

The device used to sense signals from the tracked source 402 is the sameultrasonic probe 408 (ideally a 2D probe for 3D tracking) and scannerthat are used to make the intra-operative ultrasound anatomical images416.

The scanner triggers emission of sound from the tracked source 402 withits line trigger (which is designed to be fired upon emission of eachbeam) or frame trigger 426 (which is designed to be fired upon emissionof each frame), propagation of sound then occurring from the source tothe individual elements 104 of the imaging array 102.

Alternatively, the tracked source 402 can be the one that triggers imageacquisition by the ultrasound scanner; this might be desirable in thecase where the duty cycle and on/off times of the source on the surgicaltool 404 have been optimized for best treatment safety and efficacy (inthe case where the tracked source is actually used for treatment). Ineffect then, and ultrasound device is configured for an ultrasoundscanner triggering, by a line trigger or by a frame trigger, emission ofsound from the source 402 and/or for the source triggering the scanneractive for image acquisition.

The most important modification that has to be made to the ultrasoundscanner for tracking the source 402 is to adjust its receive beamformingdelays, e.g., [r(P)+d(i,P)]/c as in FIG. 1, to account for the one-wayonly ultrasound propagation (from the tracked source to the probe 408).In FIG. 4, this is implemented as a one-way beamformer 428 whosefunction is separate from the pulse-echo receive beamformer of thescanner's beamformer 430.

The ultrasound scanner alternates imaging frames (active ultrasoundemission from the imaging probe 408, the tracked source 402 on theinterventional tool 404 is turned off, and conventional two-waybeamforming is performed for pulse-echo imaging) with tracking frames422 (emission from the imaging probe is turned off, the tracked sourceon the interventional tool is turned on, one-way only beamforming isperformed). Optionally, if the tracked source 402 is designed with adifferent frequency from the imaging frequencies, there is no need toturn on/off the tracked source/imaging probe during the imaging ortracking frames: for the tracking frames 422, the temporal receivefilters are just modified to take into account the different nominalfrequency of the active source.

As already mentioned, the tracked source can be replaced by a trackedreceiver. In one such embodiment, the individual elements of theultrasound scanner are turned on one by one in a synthetic aperturefashion, as explained above in connection with the top illustration inFIG. 3. As in the tracked source embodiment, imaging frames created arealternated with tracking frames created, although, unlike in the trackedsource embodiment, the switching from one mode to the other cannot beforegone in favor of creating a distinction based on differentfrequencies.

For the embodiment shown in FIG. 5, in which the scanner likewisetransmits in synthetic aperture fashion during tracking mode, thereceived signals at the tracked receiver 202 are fed back, here by awired electrical connection 540 to the ultrasound scanner's beamformingmodule 514, and one-way beamforming 516 is performed (due toreciprocity, signals sent from individual elements 104 of the imagingarray 102 and sensed at the instrument tip are identical to signals sentfrom the instrument tip and received by the individual elements of theimaging array).

It is noted that a synthetic aperture transmit scheme could also be usedto perform pulse-echo imaging. In that case, as for the syntheticaperture with virtual transducers embodiment, the pulse-echo transmitsequence would not be affected by the tracking schemes.

As an alternative to using one-way only beamforming as in theabove-described embodiments, the tracked receiver can be localized withcomparable accuracy by focusing at the depth of the receiver. Thus, inFIG. 5, there would be no one-way only beamforming 516. In particular,the ultrasound scanner keeps sending regular imaging (focused) beams.The time from beam emission to reception by the tracked receiverindicates the depth of the receiver, e.g., r(P) in FIG. 1 if thereflector 106 were a receiver. That information is fed back to theultrasound scanner that sets the transmit focal depth at the depth ofthe tracked receiver 202 for optimal lateral resolution at that depth.The position of the imaging beam that yields the highest amplitudesensed at the tracked receiver's location corresponds to the lateral (orangular, depending on the imaging geometry) location of the trackedreceiver 202.

Referring to FIG. 6, steps which are performed automatically and withoutuser intervention include: calculating, with respect to an ultrasoundscanner, an imaging depth of an ultrasound receiver (step S610),switching transmit focal depth to that imaging depth (step S620), andissuing imaging beams (with simultaneously both a tracking function anda pulse-echo imaging function) with that imaging depth as their focaldepth (step S630). While imaging and real-time tracking of the receiver202 continue (step S640), processing returns to step S610 to continuallyupdate focal depth in accordance with the then-present position of thetracked receiver 202. Also shown in FIG. 6 is a process in which theimaging beams issued in step S630 are received (step S650) and, from thereceived beams, the receiver 202 is localized (step S660) andaccordingly represented in the displayed image 224 of the region ofinterest 206. While real-time imaging/tracking continue (step S670),processing branches back to step S650.

The result is a diffraction-limited localization accuracy for thecatheter, as with the RDT technique. However, here the transmit focaldepth has to be physically modified as the catheter advances.

FIG. 7 shows an interventional tool 710 with two transmitters 720, 730attached and located mutually apart. Having two or more transmitters or,alternatively two or more receivers, on the interventional tool 710allows reliably and precisely identifying the position and orientationof an interventional tool or catheter with respect to the surroundingtissues, which is extremely useful for visualization of the surgicalprocedure, and often a difficult task using standard ultrasound imagingalone. This affords visualizing and predicting the path of theinterventional tool so that major vessels shown, e.g., in Doppler orvessel contrast modes, are reliably and safely avoided during theintervention. The configuration in FIG. 7 of the two transmitters 720,730 on the interventional tool 710 (or alternatively receivers, asappropriate) is implementable in any of the above-mentioned embodimentsof the invention.

As mentioned above, in embodiments other than that for syntheticaperture with virtual transducers, the real-time anatomy or pulse-echoultrasound is, by frequency band or by timing, kept separate fromultrasound used by the transmitter or receiver in real-time tracking. Ifby timing, acquisition of tracking frames 422 alternates withacquisition of imaging frames 416. As an alternative for embodimentswith transmitters, the separation can be accomplished by frequency band.The transmitters 720, 730 transmit at frequencies f1, f2 640, 650. Theseare in a frequency band different from that of the pulse-echo ultrasound760 from an ultrasound scanner 770, which emits ultrasound at frequencyf3 780.

In an aspect of the invention, it is an ultrasonic tracking method forreliably and precisely identifying the position of an interventionaltool or catheter with respect to the surrounding tissues, which is oftena difficult task using standard ultrasound imaging. For instance,according to what is proposed herein, the tip of a biopsy needle can beprecisely located, distinguishing it from points along the shaft whichare often mistaken for the tip when imaging the needle in traditionalpulse-echo B-mode. In addition, ultrasound emission from the tip of theneedle or catheter does not require reflection of the beam from theultrasound scanner, so that the needle or catheter is visible even incases where the imaging beam would be reflected away from the ultrasoundscanner or when the needle or needle is away from the imaging plane.Moreover, the proposed tracking technology will work in all ultrasoundimaging modes, including grayscale tissue imaging and contrast andDoppler flow imaging (as discussed hereinabove). With regard to imagingvascularity (e.g., vascular flows and tissue perfusion), all signalsfrom the tissue as well as reflected signals for the catheter or needleare suppressed in contrast imaging mode, so the catheter is invisible incontrast mode. Advantageously, the above-described embodiments of theinvention avoid this shortcoming.

The novel tracking methods and devices also overcome the need forelectromagnetic tracking of the surgical tool (thereby reducing theamount of equipment to be included in the operating room) and all theassociated calculations that have to be done for calibration of trackingand co-registration with intra-operative images. In embodiments of thepresent invention, the position of the interventional tool isautomatically co-registered with intra-operational ultrasound. This newtracking approach is cost effective and conveniently compatible with alarge base of ultrasound scanners available in hospitals.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. For example, ones of the variousmethods and devices according to the invention which, as indicatedabove, are adaptable to either a tracked source or a tracked receivercan be combined into a single embodiment by, for example, repeatingmethods steps but for the other case, or, for apparatus, providing, in asingle device, the elements for implementing both cases. In the claims,any reference signs placed between parentheses shall not be construed aslimiting the claim. Use of the verb “to comprise” and its conjugationsdoes not exclude the presence of elements or steps other than thosestated in a claim. The article “a” or “an” preceding an element does notexclude the presence of a plurality of such elements. The invention maybe implemented by means of hardware comprising several distinctelements, and by means of a suitably programmed computer having acomputer readable storage medium and/or by means of an integratedcircuit having a machine-accessible storage medium. The mere fact thatcertain measures are recited in mutually different dependent claims doesnot indicate that a combination of these measures cannot be used toadvantage.

What is claimed is:
 1. A medical system for locating an objectcomprising: an ultrasound imaging device configured to provide imagingbeams insonifying a region of interest; a receive device configured toreceive the imaging beams and being located on the object; and aprocessor configured to process digital signal outputs by the receivedevice to locate the object in the region of interest; wherein theprocessor has a receive beamformer configured to perform one-way onlybeamforming of output signals from the receive device.
 2. The medicalsystem of claim 1, wherein the processor is configured to processdigital signals using at least one of signal processing, beamforming,computer vision and pattern recognition methods.
 3. The medical systemof claim 1, wherein one-way beamforming includes retrospectivelyfocusing the imaging beams at all depths in the region of interest. 4.The medical system of claim 1, wherein the processor is furtherconfigured for use with three-dimensional imaging.
 5. The ultrasounddevice of claim 1, wherein the processor is configured for real-timetracking of the object within the region of interest.
 6. A method forlocating an object within a region of interest via a medical systemcomprising acts of: with a receive device, detecting one or more imagingbeams insonifying the region of interest, the receive device beingaffixed to the object; receiving digital signal outputs by a processorfrom the receive device; processing the digital signal outputs to locatethe object in the region of interest, wherein the processing actcomprises signal processing of one-way only output signals from thereceive device.
 7. The method of claim 6, wherein the act of processingthe digital signal outputs to locate the object in the region ofinterest comprises at least one of signal processing, beamforming,computer vision and pattern recognition methods.
 8. The method of claim6, wherein the act of performing one-way only beamforming of outputsignals from the receive device comprises retrospectively focusing theimaging beams at all depths in the region of interest.
 9. The method ofclaim 6, wherein the region of interest comprises a three-dimensionalspace.
 10. The method of claim 6, wherein the act of processing thedigital signal outputs to locate the object in the region of interestcomprises real-time tracking of the object within the region ofinterest.